Magnet assembly



Dec. 12, 1967 R. J. PARKER MAGNET AS SEMBLY Filed June 10, 1964 Eai.

INVENTOR. ROLLIN J. PARKER ,4 r TOR-NE) 3,358,089 Patented Dec. 12, 19673,358,089 MAGNET ASSEMBLY RollinJ. Parker, Greenville, Mich., assignorto General Electric Company, a corporation of New York Filed June 10,1964, Ser. No. 374,050 6 Claims. (Cl. 179-117) ABSTRACT OF THEDISCLGSURE A permanent magnet loudspeaker in which the permanent magnetassembly comprises a cup-shaped per- "manent'magnet and a cylindricalpole piece integrally "cast within the cup-shaped magnet. The crystalstructure of themagnet is oriented in the direction of magnetic"orientation. Such directional grain magnets are produced by casting'themolten alloy against a cylindrical chill plate-Upon completion ofsolidification of the magnetic 'alloy, the permanent magnet and chillplate are an integral unit,with the chill plate acting as pole piece.

This invention relates to a permanent magnet assembly adapted'for use ina loudspeaker and to a process for producing such permanent magnetassembly.

Much progress has been made in the direction ofminiaturizing-loudspeaker magnets and, in particular, in producingloudspeakers of reduced depth without sacrifice 'of' soundqualityfHowever, further miniaturization of permanent'magnet loudspeakers facesan obstacle inherent in the design of conventional loudspeakersthemselvesr-To maintain a'given air gap density across the -'voice coilof a speaker requires a magnet of a corresponding predeterminedstrength. If the'length'of a magnet' is shortened to decrease the depthof the'speaker,

.of higher air gap densities than any other permanent -rnagnet assemblypresently known.

' It is anadditional object of this invention to provide a permanentmagnet assembly in which the diameter of a-themagnetmay be variedthrough a wide range, permit- .ting greatfiexibility. in the design ofpermanent magnet speakers, including a reduction in their depth.

It is stillan additional object of this invention to providea processfor producing a permanent magnet assem- .bly by casting: a directionalgrain permanent magnet against a chill plate whereby, uponsolidification, the chill plateand cast permanent magnet form a unitarystructure, v

These an d other objects of the invention are achieved gin a'permanentmagnet assemblyin which the position .-of .the permanent magnet and .thereturn path are, in effect, reversed. A cup-shaped permanent magnet,oriented ra- .dially, that is in a direction along the path of themagnetic circuit of the speaker magnet assembly, is adapted ,tofitexternally around the voice coil and a cylindrical or similarlyshaped pole piece cast integrally within the cup-shaped permanent magnetis adapted to fit within the voice coil. The direction of orientation isreferred to as "radialwbecause it is oriented generally along a pathfrom the center to the outer extremity or lip of the cup- .shapedmagnet. It is intended to distinguish such orientat i on fromorientation in a single axial or single diametric United States PatentCH direction. The permanent magnet is cup-shaped and is radiallyoriented so that its preferred direction of magnetization lies along apath generally following the flow of magnetic flux to and from the airgap. One pole of the magnet is at the central portion of the cup-shapedmagnet in contact with the integral pole piece. The second pole is atthe opposite extremity or lip of the cupshaped magnet.

The invention will be more clearly understood from the followingdescription taken in connection with the accompanying drawing in which:

FIG. 1 is'an elevational crosssectional view of a loudspeakerincorporating a permanent magnet assembly illustrating one embodiment ofthe invention;

FIG. 2 is an enlarged fragmentary view of the speaker of FIG. 1, showingwith greater clarity the structure of the permanent magnet assembly;

FIG. 3 is a bottom elevational view of the permanent magnet assembly ofFIGS. 1 and 2;

FIG. 4 is a perspective view of the permanent magnet assembly shown inFIGS. 1 and 2; and

FIGS. 5 and 6 are elevational cross-sectional views of other embodimentsof speaker magnet assemblies of the invention.

Referring to FIGS. 1 and 2, the loudspeaker comprises a supporting frameor basket 1, at the outer edge of which a flange 2 supportsfrusto-couical diaphragm 3 by means of a securing ring 4. A flexiblespider 5 is secured at its outer periphery to basket 1. A voice coil 6is wound on a voice coil form 7 and is coupled to diaphragm 3. Apermanent magnet assembly 8 is secured by rivets 9 to the supportingframe of the speaker.

As can be most clearly seen in FIGS. 2 through 4., the permanent magnetassembly 8 comprises a cup-shaped permanent magnet 10 and a soft steelcylindrical pole piece 11, located centrally within the cup-shapedpermanent magnet. The cup-shaped magnet has a rim 12 at one end thereofand an aperture 13 in the base portion at the other end thereof. Themagnetic circuit is completed by a soft steel washer 14 and disc 15.Washer 14 contains three recesses 16, each of which receives one of therivets 9 which, in combination with frame 1 and recessed surfaces 17 and18 in the side and bottom surfaces, respectively, of the permanentmagnet holds the washer in place against the rim orlip 12 of thepermanent magnet and secures the magnet assembly to the frame of thespeaker. The disc 15 may be held on the pole'piece 11 eithermagnetically or by welding, brazing or other conventional means ofattachment.

As magnetic flux moves across the air gap 19 to washer 14 (or in thereverse direction to disc 15), the flux traverses voice coil 6 whichdrives diaphragm 3 to which it is coupled in accordance with the varyingelectrical signals impressed on the coil. As illustrated by the arrowsin FIG. 2, the magnetic flux .follows substantially the same path as themagnetic flux circuit of a permanent magnet speaker. It can thus be seenthat the return path is, in effect, eliminated in the present inventionby making the return path itself a permanent magnet.

The magnetic circuit of the speaker is completed by the simple additionof an annular washer at the rim of the permanent magnet and a disc onthe pole piece. It is preferable that the depth of cup-shaped magnet 10be substantially the same as the depth of pole peice 11 so that theupper surface of the pole piece and of rim 12 are in a single plane asillustrated in the drawings. It is possible to eliminate disc 15 byusing a pole piece having In conventional speakers, to of the magneticpotential (H) is unavoidably lost in the steel return yoke. In thepresent invention, the yoke and joints are essentially eliminated sothat substantially all magnetic potential is used in the air gapsurrounding the voice coil. Moreover, the diameter of the voice coildoes not limit the diameter of the permanent magnet so that greatflexibility is possible in the design of the permanent magnet assembly.The result is that a speaker of very shallow depth is made possible byincreasing the diameter of the permanent magnet to virtually anydimension desired to compensate for the decrease in its length resultingfrom a shallower speaker design. Miniaturization of speaker magnets withrespect to depth is thereby made possible without loss of total magneticenergy or speaker performance.

An additional advantage of the invention is that it makes possible theuseof varying diameter voice coils with a single permanent magnetassembly. This is accomplished by simply attaching a disc of smaller orgreater diameter and a washer of correspondingly decreased or increasedinternal diameter to the permanent magnet assembly. This feature of theinvention is illustrated in FIGS. 5 and 6. In FIG. 5, permanent magnetassembly 8 is made adaptable for use with a voice coil of smallerdiameter than that of FIG. 2, by simply utilizing a disc 20 of reduceddiameter together with a washer 21 of decreased internal diameter.Washer 21 is provided with a rim 22 of greater depth than the body ofthe washer so that clearance is provided at the air gap between polepiece 11 and washer 21. In FIG. 6, the permanent magnet assembly is madeadaptable for use with a voice coil of larger diameter by use of a disc23 of larger diameter and a washer 24 of increased internal diameter.Thus a single permanent magnet assembly may be used, for example, forboth a highfrequency tweeter requiring a smaller diameter voice coil(FIG. 5) and a low-frequency woofer requiring a larger diameter voicecoil (FIG. 6).

A still additional advantage of the present invention resides in theutilization of the chill plate, used in producing the radially orienteddirectional grain permanent magnet, as the flux collecting component orpole piece in the final speaker assembly. Highest energy products inpermanent magnets are obtained by orienting the crystal structure of themagnets in the direction of their magnetic orientation. Such directionalgrain magnets are ideally suitable for use in loudspeakers where higherenergy products result in higher speaker performance. Orientation of thecrystal structure of such magnets takes place by casting the moltenmagnetic alloy against a chill plate which extracts heat from the moltenalloy during solidification from one direction in the casting andresults in the growth of a columnar grain structure in the preferreddirection of magnetization. The production of such directional grainpermanent magnets is more fully disclosed in U.S. Patent 2,578,407, thedisclosure of which is hereby incorporated by reference.

In the present invention, the chill plate itself becomes the pole pieceof the permanent magnet assembly. Pole piece 11 (FIGS. 1 and 2) is thechill plate used in preparing the directional grain permanent magnet.Upon completion of solidification of the magnetic alloy, the permanentmagnet and chill plate form an integral unit forming the permanentmagnet assembly of the invention, with the chill plate now acting as apole piece.

In an illustrative process for practicing the present invention, amolten alloy of the type commonly known as Alnico 5 and consisting.essentially of about 8% aluminum, 14% nickel, 24% cobalt, 3% copper,balance substantially iron, is poured into a cavity mold correspondingin shape to the cup-shape of the permanent magnet. The mold cavities arearranged so that solidification of the cast material by loss of heat toor through the side walls of the cavity is held to a minimum. A softsteel insert plug, placed centrally in the cup-shaped mold, and incontact with the molten alloy at the central aperture thereof, acts as ametal chill and removes the heat liberated during solidification of thealloy. When solidified, the magnetic alloy and the centrally disposedmetal chill form an integral assembly with the metal chill becoming thepole piece.

The directional or anisotropic properties of the permanent magnet areproduced by subjecting the solid permanent magnet castings to a heattreatment in a magnetic field wherein the direction of theheat-treatingfield is made to coincide with the direction of finalmagnetization, Le, a radial direction along the path of the magneticcircuit. Such magnets exhibit magnetic properties which are appreciablyhigher in one direction than in a direction perpendicular to thepreferred direction. The process for so directionalizing the magnets isdescribed, for example, in Jonas U.S. Patent 2,295,082.

The directional grain permanent magnets to which the present inventionis particularly directed are of the ironcobalt-nickel-aluminum alloytype commonly known as Alnico 5. While directional grain Alnico. 5permanent magnets are the preferred magnetic materials useful in thepractice of the present invention, other permanent magnetic materialsmay obviously be used so long as they are useful in the production ofhigh energy permanent magnet loudspeakers.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A permanent magnet assembly adapted for use in a permanent magnetloudspeaker comprising acup-shaped directional grain permanent magnethaving arim at a first end thereof and a hole at the opposite basel e'ndthereof and a pole piece cast integrally within the hole in the base ofthe cup-shaped magnet, the magnet being oriented crystallographicallyand magnetically in a direction along the contour of the cup-shapedmagnet from the hole in the base to the rim thereof, said direction oforientation adapted to coincide with the path of the magnetic circuit insaid loudspeaker.

2. The permanent magnet assembly of claim- 1 in which the permanentmagnet is Alnico 5.

3. A permanent magnet assembly adapted for use in a permanent magnetloudspeaker comprising a cup+sha=ped directional grain permanent magnethaving a rim at a first end thereof and a hole at the opposite base endthereof and a cylindrical pole piece cast integrally and cent-rallywithin the hole in the base of the cup-shaped magnet, the depth of saidcylindrical pole piece being substantially the same as the depth of saidcup-shaped permanent magnet, the magnet being orientedcrystallographicallyandmagnetically in a direction along the contour ofthe cupshaped magnet from the hole in the base to the rim thereof, saiddirection of orientationadapted to coincide with the path of themagnetic circuit in said loudspeaker.

4. In a permanent magnet loudspeaker, a permanent magnetic assemblycomprising a cup-shaped directional grain permanent magnet having a rimatone end thereof and a hole at the opposite base end thereof and a polepiece cast integrally within the hole in the base ofthe cupshapedmagnet, the magnet being oriented crystallographically and magneticallyin a direction along the contour of the cup-shaped magnet from the holein the base to the rim thereof, said direction of orientation adapted tocoincide with the path of the magnetic circuit in said loudspeaker, awasher mounted in contact with the rim of said cup-shaped magnet, and apole piece element mounted on the pole piece of said permanent magnetassembly, said permanent magnet assembly, washer and pole piece elementtogether forming the complete magnetic circuit for said loudspeaker.

5. In a permanent magnet loudspeaker, a permanent magnet assemblycomprising a cup-shaped directional grain permanent magnet having a rimat one end thereof and a whole at the opposite base end thereof and apole piece cast integrally and centrally within the hole. in the base ofthe cup-shaped magnet, the magnet being oriented in a direction'alongthe contour of the cup-shaped magnet from the hole in the base to therim thereof, said direction of orientation adapted to coincide with thepath of the magnetic circuit in said loudspeaker, an annular washermounted in contact with the rim of said cup-shaped magnet, and adisc-shaped pole piece element mounted on the cylindrical pole piece ofsaid permanent magnet assembly, said disc-shaped pole piece having adiameter less than the diameter of said cylindrical pole piece, saidpermanent magnet assembly, washer, and pole piece element togetherforming the complete magnetic circuit for said loudspeaker.

6. In a permanent magnet loudspeaker, the permanent shaped pole pieceelement has a diameter greater than the diameter of said cylindricalpole piece.

References Cited UNITED STATES PATENTS KATHLEEN H. CLAFFY, PrimaryExaminer.

magnet assembly set forth in claim 5 in which the disc- 15 MCGILL-Assistant Examiner-

1. A PERMANENT MAGNET ASSEMBLY ADAPTED FOR USE IN A PERMANENT MAGNETLOUDSPEAKER COMPRISING A CUP-SHAPED DIRECTIONAL GRAIN PERMANENT MAGNETHAVING A RIM AT A FIRST END THEREOF AND A HOLE AT THE OPPOSITE BASE ENDTHEREOF AND A POLE PIECE CAST INTEGRALLY WITHIN THE HOLE IN THE BASE OFTHE CUP-SHAPED MAGNET, THE MAGNET BEING ORIENTED CRYSTALLOGRAPHICALLYAND MAGNETICALLY IN A DIRECTION ALONG THE CONTOUR OF THE CUP-SHAPEDMAGNET FROM THE HOLE IN THE